Optical transmission device

This application is a National Stage Entry of International Application No. PCT/JP2017/006387, filed Feb. 21, 2017, which claims priority from Japanese Patent Application No. JP 2016-038162, filed Feb. 29, 2016. The entire contents of the above-referenced applications are expressly incorporated herein by reference.

The present invention relates to an optical transmission device, and particularly relates to an optical transmission device used in an optical communication network.

In association with an increase in demand for a wideband multimedia communication service, such as the Internet and image distribution, introduction of an optical fiber communication system that covers a long distance and that has a large capacity has been progressing in trunk line networks and metro access networks. In such an optical communication system using optical fibers, it is important to increase transmission efficiency per optical fiber. For this reason, wavelength division multiplex (WDM) communication, in which a plurality of optical signals having different wavelengths are multiplexed and transmitted, are widely used.

Optical transmission devices are required to perform transmission over a longer distance than that conventionally covered and to have operational flexibility because of an increase in information communication traffic in recent years, and research and development in elastic optical technology has been conducted.

PTL 1 and NPL 1 disclose technologies relating to optical transmission devices that are capable of autonomously changing a pass band for optical signals while being in operation in an optical network using a WDM technology.

Patent Literature

[PTL 1] JP 2015-19289 A

Non Patent Literature

[NPL 1] Shin Kamei et al., “N×N Cyclic-Frequency Router With Improved Perfomance Based on Arrayed-Waveguide Grating”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Sep. 15, 2009, VOL. 27, NO. 18, p 4097-4104

Technical Problem

As described in the background art, an optical transmission device used in an optical communication network is required to perform long distance transmission and to have operational flexibility. For example, a colorless, directionless and contentionless reconfigurable optical add drop multiplexer (CDC-ROADM) is a system having high flexibility. However, the CDC-ROADM has a problem in that use of a wavelength selective switch (WSS) causes the number of optical filters to be passed per node to increase, influence of band narrowing due to optical filters to increase, and transmission characteristics to deteriorate.

Although methods to solve the problem relating to operational flexibility include a configuration disclosed in NPL 1, the configuration cannot solve the above-described problem relating to band narrowing. In addition, there is a problem in that the configuration is incapable of supporting a flexible grid, which is indispensable for achieving an elastic network.

In consideration of the above-described problems, an object of the present invention is to provide an optical transmission device that, while suppressing optical signals from being trimmed because of band narrowing due to optical filters, achieves flexibility of optical communication such as wavelength reutilization and that supports a flexible grid.

Solution to Problem

An optical transmission device according to the present invention includes a cyclic AWG that filters respective optical signals inputted to each input port. The respective optical signals are constituted so that a plurality of wavelength-multiplexed signals can be allocated within one channel band, the respective optical signals are filtered in channel units, and the pass-band width of each of the input ports of the cyclic AWG corresponds to the bandwidth of the channel.

Advantageous Effects of Invention

The invention enables an optical transmission device to be provided that, while suppressing optical signals from being trimmed because of band narrowing due to optical filters, achieves flexibility of optical communication such as wavelength reutilization and that supports a flexible grid.

FIG. 1 is a diagram for a description of an outline of an optical transmission device according to the present invention;

FIG. 2 is a diagram for a description of an optical transmission device according to a first example embodiment;

FIG. 3 is a diagram for a description of channel bandwidths used by the optical transmission device according to the first example embodiment;

FIG. 4 is a diagram for a description of details of a cyclic AWG used by the optical transmission device according to the first example embodiment;

FIG. 5 is a diagram for a description of an optical transmission device according to a comparative example;

FIG. 6 is a diagram for a description of channel bandwidths used by the optical transmission device according to the comparative example;

FIG. 7 is a diagram for a description of another configuration example of the optical transmission device according to the first example embodiment;

FIG. 8 is a diagram for a description of a problem in a second example embodiment; and

FIG. 9 is a diagram for a description of pass-band widths of a cyclic AWG that an optical transmission device according to the second example embodiment includes.

Hereinafter, example embodiments of the present invention will be described with reference to the drawings.

First, an outline of the example embodiments of the present invention will be described. FIG. 1 is a diagram for a description of an outline of an optical transmission device according to each example embodiment. As illustrated in FIG. 1 , the optical transmission device according to each example embodiment includes a cyclic arrayed waveguide grating (AWG) that filters respective optical signals (Ch 11 to Ch 33 ) inputted to respective input ports P 1 to P 3 . As illustrated in FIG. 1 , the optical signals Ch 11 to Ch 13 , Ch 21 to Ch 23 , and Ch 31 to Ch 33 are inputted to the input ports P 1 , P 2 , and P 3 , respectively.

The respective optical signals are constituted so that a plurality of wavelength-multiplexed signals can be allocated within one channel band (see FIG. 3 ). In addition, the respective optical signals are filtered in channel units. In the cyclic AWG illustrated in FIG. 1 , the optical signals Ch 11 , Ch 32 , and Ch 23 , out of the respective optical signals (Ch 11 to Ch 33 ), are output from an output port. In this configuration, the pass-band width of each of the input ports of the cyclic AWG corresponds to the bandwidth of a channel.

That is, in the optical transmission device according to each example embodiment, expansion of the pass-band width of each of the input ports P 1 to P 3 of the cyclic AWG to the bandwidth of a channel enables the cyclic AWG to respectively filter the optical signals (Ch 11 , Ch 12 , Ch 13 , . . . , and Ch 33 ) having a predetermined bandwidth.

The filtering capability in conjunction with a capability of arbitrarily setting the number and the bandwidths of a plurality of wavelength-multiplexed signals allocated within each channel (see FIG. 3 ) enables the optical transmission device to flexibly set optical signals to be transmitted. Hereinafter, the optical transmission device according to each example embodiment will be described in detail.

<First Example Embodiment>

FIG. 2 is a diagram for a description of an optical transmission device according to a first example embodiment. As illustrated in FIG. 2 , an optical transmission device 1 according to the present example embodiment is configured using a plurality of beam splitters BS_ 1 to BS_ 3 and a plurality of cyclic AWGs AWG_ 1 to AWG_ 3 . The optical transmission device 1 according to the present example embodiment may be used in an optical node, and more specifically an optical cross-connect node, in an optical communication network.

As illustrated in FIG. 2 , the input sides of the beam splitters BS_ 1 to BS_ 3 are connected to three input side paths IN_# 1 to IN_# 3 , respectively. Optical signals outputted from the beam splitters BS_ 1 to BS_ 3 are supplied to the cyclic AWGs AWG_ 1 to AWG_ 3 , which are arranged at the succeeding stage of the beam splitters BS_to BS_ 3 . Each of the beam splitters BS_ 1 to BS_ 3 is a beam splitter with one input and three outputs (1×3), and each of the cyclic AWGs AWG_ 1 to AWG_ 3 is a cyclic AWG with three inputs and one output (3×1). The output ports of the cyclic AWGs AWG_ 1 to AWG_ 3 are connected to output side paths OUT_# to OUT_# 3 , respectively.

Each of the beam splitters BS_ 1 to BS_ 3 outputs respective optical signals that are inputted to the beam splitter to an input port of each of the cyclic AWGs AWG_ 1 to AWG_ 3 . Specifically, the beam splitter BS_ 1 splits optical signals Ch 11 , Ch 12 , and Ch 13 supplied from the input side path IN_# 1 and supplies the optical signals Ch 11 , Ch 12 , and Ch 13 to an input port P 1 of the cyclic AWG AWG_ 1 , an input port P 2 of the cyclic AWG AWG_ 2 , and an input port P 3 of the cyclic AWG AWG_ 3 . Similarly, the beam splitter BS_ 2 splits optical signals Ch 21 , Ch 22 , and Ch 23 supplied from the input side path IN_# 2 and supplies the optical signals Ch 21 , Ch 22 , and Ch 23 to an input port P 2 of the cyclic AWG AWG_ 1 , an input port P 3 of the cyclic AWG AWG_ 2 , and an input port P 1 of the cyclic AWG AWG_ 3 . Still similarly, the beam splitter BS_ 3 splits optical signals Ch 31 , Ch 32 , and Ch 33 supplied from the input side path IN_# 3 and supplies the optical signals Ch 31 , Ch 32 , and Ch 33 to an input port P 3 of the cyclic AWG AWG_ 1 , an input port P 1 of the cyclic AWG AWG_ 2 , and an input port P 2 of the cyclic AWG AWG_ 3 .

Each of the cyclic AWGs AWG_ 1 to AWG_ 3 filters (selects) respective optical signals inputted to each of the input ports P 1 to P 3 of the cyclic AWG and outputs the filtered optical signals to the output side path connected thereto. Specifically, the cyclic AWG AWG_ 1 outputs the optical signals Ch 11 , Ch 32 , and Ch 23 out of the optical signals inputted to the input ports P 1 to P 3 thereof to the output side path OUT_# 1 . In addition, the cyclic AWG AWG_ 2 outputs the optical signals Ch 21 , Ch 12 , and Ch 33 out of the optical signals inputted to the input ports P 1 to P 3 thereof to the output side path OUT_# 2 . Still in addition, the cyclic AWG AWG_ 3 outputs the optical signals Ch 31 , Ch 22 , and Ch 13 out of the optical signals inputted to the input ports P 1 to P 3 thereof to the output side path OUT_# 3 .

In the above configuration, the respective optical signals Ch 11 , Ch 12 , and Ch 13 , which pass through the input side path IN_# 1 , are optical signals in frequency bands different from one another. The same applies to the respective optical signals Ch 21 , Ch 22 , and Ch 23 , which pass through the input side path IN_# 2 , and the respective optical signals Ch 31 , Ch 32 , and Ch 33 , which pass through the input side path IN_# 3 .

Meanwhile, since the optical signal Ch 11 in the input side path IN_# 1 , the optical signal Ch 21 in the input side path IN_# 2 , and the optical signal Ch 31 in the input side path IN_# 3 are optical signals in an identical frequency band, the optical signals are, in order to achieve WDM communication, required to be set not to be outputted to an identical output side path. Similarly, since the optical signal Ch 12 in the input side path IN_# 1 , the optical signal Ch 22 in the input side path IN_# 2 , and the optical signal Ch 32 in the input side path IN_# 3 are optical signals in an identical frequency band, the optical signals are, in order to achieve WDM communication, required to be set not to be outputted to an identical output side path. Still similarly, since the optical signal Ch 13 in the input side path IN_# 1 , the optical signal Ch 23 in the input side path IN_# 2 , and the optical signal Ch 33 in the input side path IN_# 3 are optical signals in an identical frequency band, the optical signals are, in order to achieve WDM communication, required to be set not to be outputted to an identical output side path.

In the optical transmission device 1 according to the present example embodiment, the respective optical signals (Ch 11 , Ch 12 , Ch 13 , . . . , and Ch 33 ) are routed in channel units. In addition, the respective optical signals are constituted so that a plurality of wavelength-multiplexed signals can be allocated within the band of one channel.

FIG. 3 is a diagram for a description of bandwidths of channels used by the optical transmission device 1 according to the present example embodiment. As illustrated in FIG. 3 , in the optical transmission device 1 according to the present example embodiment, a plurality of wavelength-multiplexed signals (optical signals) 12 can be allocated within one channel band 11 . In FIG. 3 , a case where the bandwidths of the respective channels Ch 11 , Ch 12 , and Ch 13 are set at 150 GHz and three optical signals with a bandwidth of 50 GHz are allocated in the channel Ch 11 is illustrated as an example. In addition, a case where four optical signals with a bandwidth of 37.5 GHz are allocated in the channel Ch 12 is illustrated. Still in addition, a case where an optical signal with a bandwidth of 100 GHz and an optical signal with a bandwidth of 50 GHz are allocated in the channel Ch 13 is illustrated. Note that the examples illustrated in FIG. 3 are just examples and the bandwidths of the respective channel bands 11 may be determined arbitrarily. Note also that the bandwidths and the number of optical signals 12 to be allocated in the band of each channel may be determined arbitrarily.

In the optical transmission device 1 according to the present example embodiment, the pass-band width of each of the input ports P 1 to P 3 of each of the cyclic AWGs AWG_ 1 to AWG_ 3 (that is, the bandwidth of a filter) is set so as to correspond to the bandwidth of each of the channels (Ch 11 , Ch 12 , Ch 13 , . . . , and Ch 33 ). In other words, expansion of the pass-band width of each of the input ports P 1 to P 3 of each of the cyclic AWGs AWG_ 1 to AWG_ 3 to the bandwidth of a channel enables optical signals each having a predetermined bandwidth (that is, the channels Ch 11 , Ch 12 , Ch 13 , . . . , and Ch 33 ) to be respectively filtered. For example, filtering the channel Ch 11 in channel units enables three optical signals 12 each of which has a bandwidth of 50 GHz to be filtered in an identical direction.

When described in detail using FIG. 4 , for example, the optical signals Ch 11 , Ch 12 , and Ch 13 , Ch 21 , Ch 22 , and Ch 23 , and Ch 31 , Ch 32 , and Ch 33 are supplied to the input ports P 1 , P 2 , and P 3 of the cyclic AWG AWG_ 1 , respectively. On this occasion, setting the pass-band width of each of the input ports P 1 to P 3 at 150 GHz enables an optical signal with a bandwidth per channel of 150 GHz to be filtered.

For example, making the pass band and the center frequency of the input port P 1 of the cyclic AWG AWG_ 1 coincide with the frequency band and the center frequency of the optical signal Ch 11 , respectively, enables only the optical signal Ch 11 out of the optical signals Ch 11 , Ch 12 , and Ch 13 supplied to the input port P 1 to be passed (that is, enables the optical signals Ch 12 and Ch 13 to be removed). Similarly, making the pass band and the center frequency of the input port P 2 of the cyclic AWG AWG_ 1 coincide with the frequency band and the center frequency of the optical signal Ch 23 , respectively, enables only the optical signal Ch 23 out of the optical signals Ch 21 , Ch 22 , and Ch 23 supplied to the input port P 2 to be passed. Still similarly, making the pass band and the center frequency of the input port P 3 of the cyclic AWG AWG_ 1 coincide with the frequency band and the center frequency of the optical signal Ch 32 , respectively, enables only the optical signal Ch 32 out of the optical signals Ch 31 , Ch 32 , and Ch 33 supplied to the input port P 3 to be passed.

On this occasion, the pass-band width and the center frequency interval of each of the input ports P 1 to P 3 of the cyclic AWG AWG_ 1 are 150 GHz and 150 GHz, respectively.

For example, the pass-band width of each of the input ports P 1 to P 3 of each of the cyclic AWGs AWG_ 1 to AWG_ 3 may be set at m (m is a positive real number not less than 2) times of an adjacent frequency interval of a plurality of wavelength-multiplexed signals. For example, when it is assumed that the adjacent frequency interval of wavelength-multiplexed signals is 37.5 GHz and m=4, the pass-band widths of the input ports P 1 to P 3 are 150 GHz (see Ch 12 in FIG. 3 ). In this case, when it is assumed that communication with a capacity of 100 Gbps per optical signal is performed, it is possible to achieve communication with a capacity of 400 Gbps because four optical signals with a bandwidth of 37.5 GHz can be allocated within one channel.

In addition, when, for example, it is assumed that the adjacent frequency interval of wavelength-multiplexed signals is 50 GHz and m=3, the pass-band widths of the input ports P 1 to P 3 are 150 GHz (see Ch 11 in FIG. 3 ). In this case, when it is assumed that communication with a capacity of 100 Gbps per optical signal is performed, it is possible to achieve communication with a capacity of 300 Gbps because three optical signals with a bandwidth of 50 GHz can be allocated within one channel. Still in addition, when, for example, it is assumed that the adjacent frequency interval of wavelength-multiplexed signals is 75 GHz and m=2, the pass-band widths of the input ports P 1 to P 3 are also 150 GHz. The above-described examples are just examples, and an optimal signal interval and an optimal additional bandwidth of the AWG (m times of the signal interval) are determined based on the modulation method and the capacity of involved signals.

As described above, the optical transmission device 1 according to the present example embodiment is configured so that a plurality of wavelength-multiplexed signals can be allocated within the band of one channel. In the above configuration, the number and the bandwidths of a plurality of wavelength-multiplexed signals allocated in the band of one channel may be set arbitrarily (flexible grid).

In addition, the configuration of the optical transmission device 1 according to the present example embodiment using cyclic AWGs enables the number of optical filters to be passed per node to be reduced and influence of band narrowing due to optical filters to be reduced when compared with a case where wavelength selective switches (WSS) are used.

Still in addition, the use of the cyclic AWGs AWG_ 1 to AWG_ 3 for filtering each optical signal in the optical transmission device 1 according to the present example embodiment enables signals in an identical frequency band to be avoided from being outputted to an identical output side path.

Accordingly, the invention according to the present example embodiment enables an optical transmission device to be provided that, while suppressing optical signals from being trimmed because of band narrowing due to optical filters, achieves flexibility of optical communication such as wavelength reutilization and that supports a flexible grid.

Hereinafter, advantageous effects of the present example embodiment will be described in detail.

FIG. 5 is a diagram for a description of an optical transmission device according to a comparative example. As illustrated in FIG. 5 , an optical transmission device 101 according to the comparative example is configured using a plurality of AWGs AWG_ 1 to AWG_ 13 that are arranged at the preceding stage and a plurality of AWGs AWG_ 21 to AWG_ 23 that are arranged at the succeeding stage. Each of the AWGs AWG_ 11 to AWG_ 13 is an AWG with one input and three outputs (1×3), and each of the AWGs AWG_ 21 to AWG_ 23 is an AWG with three inputs and one output (3×1).

The AWG AWG_ 11 branches optical signals λ 11 to λ 13 that are supplied from an input side path IN_# 1 and outputs the optical signal λ 11 , λ 12 , and λ 13 to a port P 1 of the AWG AWG_ 21 , a port P 2 of the AWG AWG_ 22 , and a port P 3 of the AWG AWG_ 23 , respectively. In addition, the AWG AWG_ 12 branches optical signals λ 21 to λ 23 that are supplied from an input side path IN_# 2 and outputs the optical signal λ 21 , λ 22 , and λ 23 to a port P 1 of the AWG AWG_ 22 , a port P 2 of the AWG AWG_ 23 , and a port P 3 of the AWG AWG_ 21 , respectively. Still in addition, the AWG AWG_ 13 branches optical signals λ 31 to λ 33 that are supplied from an input side path IN_# 3 and outputs the optical signal λ 31 , λ 32 , and λ 33 to a port P 1 of the AWG AWG_ 23 , a port P 2 of the AWG AWG_ 21 , and a port P 3 of the AWG AWG_ 22 , respectively.

The AWG AWG_ 21 multiplexes and outputs the optical signals λ 11 , λ 32 , and λ 23 that are supplied to the ports P 1 , P 2 , and P 3 , respectively, to an output side path OUT_# 1 . In addition, the AWG AWG_ 22 multiplexes and outputs the optical signals λ 21 , λ 12 , and λ 33 that are supplied to the ports P 1 , P 2 , and P 3 , respectively, to an output side path OUT_# 2 . Still in addition, the AWG AWG_ 23 multiplexes and outputs the optical signals λ 31 , λ 22 , and λ 13 that are supplied to the ports P 1 , P 2 , and P 3 , respectively, to an output side path OUT_# 3 .

FIG. 6 is a diagram for a description of bandwidths of channels used by the optical transmission device according to the comparative example. As illustrated in FIG. 6 , in the optical transmission device 101 according to the comparative example, one optical signal is allocated to one channel. That is, to a channel Ch 1 , one optical signal λ 11 (with a bandwidth of 50 GHz) is allocated. Similarly, to channels Ch 2 and Ch 3 , the optical signals λ 12 and λ 13 are allocated, respectively. The same applies to the other optical signals λ 21 to λ 23 and λ 31 to λ 33 .

That is, in the optical transmission device 101 according to the comparative example, channels have a one-on-one correspondence with optical signals, and, thus, a relationship between each of optical signals supplied to the input side paths IN_# 1 to IN_# 3 and each of optical signals outputted from the output side paths OUT_# 1 to OUT_# 3 is determined uniquely. In other words, a relationship between each of optical signals supplied to the input side paths IN_# 1 to IN_# 3 and each of optical signals outputted from the output side paths OUT_# 1 to OUT_# 3 is fixed. Therefore, there has been a problem in that the optical transmission device 101 cannot support a flexible grid.

By contrast, in the optical transmission device 1 according to the present example embodiment, the device is configured in such a manner that a plurality of wavelength-multiplexed signals can be allocated within the band of one channel and, further, the pass-band width of each of the input ports of a cyclic AWG is expanded so that the cyclic AWG is able to filter optical signals in channel units. This configuration enables the number and the bandwidths of a plurality of wavelength-multiplexed signals allocated in each channel to be set arbitrarily, which enables optical signals that are to be transmitted to be set flexibly. That is, since, as illustrated in FIG. 3 , the number of optical signals allocated within one channel can be varied (see the channels Ch 11 and Ch 12 ) and optical signals with bandwidths different from each other can be allocated within one channel (see the channel Ch 13 ), optical signals to be transmitted may be set flexibly.

In addition, in the optical transmission device 101 illustrated in FIG. 5 , the configuration including two stages of AWGs causes the number of filters that respective optical signals pass to be larger than that in the optical transmission device 1 illustrated in FIG. 2 . For this reason, influence of band narrowing due to optical filters increases. Means for reducing the number of filters include, for example, a configuration using an N×M AWG, which is disclosed in FIG. 2 in NPL 1. However, since overlapping of pass bands between filters as described in a second example embodiment, which will be described later, cannot be achieved due to the principle of the AWG, influence of band narrowing due to optical filters cannot be minimized.

A CDC-ROADM is a system with high flexibility because the CDC-ROADM uses active WSSes. However, due to the CDC-ROADM using WSSes, the number of optical filters to be passed per node increases and influence of band narrowing due to optical filters becomes large.

By contrast, in the optical transmission device 1 according to the present example embodiment, since the number of stages at which cyclic AWGs used therein are arranged is one, the number of filters that respective optical signals pass may be set smaller than that in the optical transmission device 101 according to the comparative example illustrated in FIG. 5 . In addition, since no WSS is used in the optical transmission device 1 according to the present example embodiment, influence of band narrowing due to the optical filters may be reduced.

The invention according to the present example embodiment, which has been described thus far, enables an optical transmission device to be provided that, while suppressing optical signals from being trimmed because of band narrowing due to optical filters, achieves flexibility of optical communication such as wavelength reutilization and that supports a flexible grid.

Note that, as to the optical transmission device 1 illustrated in FIG. 2 , a case where both the number of input side paths and the number of output side paths were three was described as an example. However, in the optical transmission device according to the present example embodiment, the number of input side paths and the number of output side paths may be a number other than three. That is, in the case of a configuration including n (n is a natural number not less than 2) input side paths IN_# 1 to IN_#n and n output side paths OUT_# 1 to OUT_#n, n beam splitters BS_ 1 to BS_n (each thereof is a beam splitter with one input and n outputs) that are connected to the input side paths IN_# 1 to IN_#n, respectively, are arranged. In addition, n cyclic AWGs AWG_ 1 to AWG_n (each thereof is a cyclic AWG with n inputs and one output) are arranged at the succeeding stage of the n beam splitters BS_ 1 to BS_n. Each of the n beam splitters BS_ 1 to BS_n outputs respective optical signals that are inputted to the beam splitter to an input port of each of the n cyclic AWGs AWG_ 1 to AWG_n. Each of the n cyclic AWGs AWG_ 1 to AWG_n filters respective optical signals inputted to each of the input ports of the cyclic AWG and outputs the filtered optical signals to the output side path connected thereto. This operation causes optical signals after filtering to be outputted from the respective cyclic AWGs AWG_ 1 to AWG_n to the output side paths OUT_# 1 to OUT_#n.

FIG. 7 is a diagram for a description of another configuration example of the optical transmission device according to the present example embodiment. In an optical transmission device 2 illustrated in FIG. 7 , a configuration when the value of the above-described n is set as n=4 is illustrated. As illustrated in FIG. 7 , the input sides of beam splitters BS_ 1 to BS_ 4 are connected to four input side paths IN_# 1 to IN_# 4 , respectively. Optical signals outputted from the beam splitters BS_ 1 to BS_ 4 are supplied to cyclic AWGs AWG_ 1 to AWG_ 4 , which are arranged at the succeeding stage of the beam splitters BS_ 1 to BS_ 4 . Each of the beam splitters BS_ 1 to BS_ 4 is a beam splitter with one input and four outputs (1×4), and each of the cyclic AWGs AWG_ 1 to AWG_ 4 is a cyclic AWG with four inputs and one output (4×1). The output ports of the cyclic AWGs AWG_ 1 to AWG_ 4 are connected to output side paths OUT_# 1 to OUT_# 4 , respectively.

Each of the beam splitters BS_ 1 to BS_ 4 outputs respective optical signals that are inputted to the beam splitter to an input port of each of the cyclic AWGs AWG_ 1 to AWG_ 4 . Specifically, the beam splitter BS_ 1 splits optical signals Ch 11 to Ch 18 supplied from the input side path IN_# 1 . The beam splitter BS_ 1 supplies the split optical signals Ch 11 to Ch 18 to an input port P 1 of the cyclic AWG AWG_ 1 , an input port P 2 of the cyclic AWG AWG_ 2 , an input port P 3 of the cyclic AWG AWG_ 3 , and an input port P 4 of the cyclic AWG AWG_ 4 .

Similarly, the beam splitter BS_ 2 splits optical signals Ch 21 to Ch 28 supplied from the input side path IN_# 2 . The beam splitter BS_ 2 supplies the split optical signals Ch 21 to Ch 28 to an input port P 2 of the cyclic AWG AWG_ 1 , an input port P 3 of the cyclic AWG AWG_ 2 , an input port P 4 of the cyclic AWG AWG_ 3 , and an input port P 1 of the cyclic AWG AWG_ 4 .

Still similarly, the beam splitter BS_ 3 splits optical signals Ch 31 to Ch 38 supplied from the input side path IN_# 3 . The beam splitter BS_ 3 supplies the split optical signals Ch 31 to Ch 38 to an input port P 3 of the cyclic AWG AWG_ 1 , an input port P 4 of the cyclic AWG AWG_ 2 , an input port P 1 of the cyclic AWG AWG_ 3 , and an input port P 2 of the cyclic AWG AWG_ 4 .

Still similarly, the beam splitter BS_ 4 splits optical signals Ch 41 to Ch 48 supplied from the input side path IN_# 4 . The beam splitter BS_ 4 supplies the split optical signals Ch 41 to Ch 48 to an input port P 4 of the cyclic AWG AWG_ 1 , an input port P 1 of the cyclic AWG AWG_ 2 , an input port P 2 of the cyclic AWG AWG_ 3 , and an input port P 3 of the cyclic AWG AWG_ 4 .

Each of the cyclic AWGs AWG_ 1 to AWG_ 4 filters respective optical signals inputted to each of the input ports of the cyclic AWG and outputs the filtered optical signals to the output side path connected thereto. Optical signals outputted from each of the cyclic AWGs AWG_ 1 to AWG_ 4 to one of the output side paths OUT_# 1 to OUT_# 4 connected to the cyclic AWG are as illustrated in FIG. 7 .

In the optical transmission device 2 illustrated in FIG. 7 , the respective optical signals Ch 11 to Ch 18 , which pass through the input side path IN_# 1 , are also optical signals in frequency bands different from one another. The same applies to the respective optical signals Ch 21 to Ch 28 , which pass through the input side path IN_# 2 , the respective optical signals Ch 31 to Ch 38 , which pass through the input side path IN_# 3 , and the respective optical signals Ch 41 to Ch 48 , which pass through the input side path IN_# 4 .

In addition, in the optical transmission device 2 illustrated in FIG. 7 , the respective optical signals (Ch 11 , Ch 12 , Ch 13 , . . . , and Ch 48 ) are also filtered in channel units. The respective optical signals are constituted so that a plurality of wavelength-multiplexed signals can be allocated within the band of one channel (see FIG. 3 ). In this case, the bandwidths of respective channels may also be determined arbitrarily.

Further, the bandwidths and the number of optical signals to be allocated in the band of each channel may also be determined arbitrarily.

<Second Example Embodiment>

Next, a second example embodiment of the present invention will be described.

First, another problem related to the invention according to the present example embodiment will be described using FIG. 8 . In the optical transmission device 1 described in the first example embodiment, a plurality of wavelength-multiplexed signals are allocated within the band of one channel (see FIG. 3 ). In order to transmit optical signals with high density (a high accommodation rate) on this occasion, it is preferable that, as illustrated in the upper drawing in FIG. 8 , there be no difference between a filter bandwidth 21 and a bandwidth of four optical signals 22 (that is, a bandwidth per channel). In the case illustrated in the upper drawing in FIG. 8 , for example, since the bandwidth of one optical signal 22 is 37.5 GHz and the number of the optical signals 22 is four, the bandwidth of the four optical signals 22 (bandwidth per channel) becomes 37.5 GHz×4=150 GHz. In this case, setting the filter bandwidth 21 at 150 GHz enables optical signals to be transmitted with high density.

However, since the shape of an optical filter is not rectangular and there is individual variation in the center frequency and the bandwidth of each optical filter, the bandwidth of filters viewed at the receiving end becomes narrower (band narrowing) as the number of filters increases. Such a characteristic of filters causes optical signals to be trimmed at both ends of the frequency band thereof and to lose some signal components therein during a transmission process, and, eventually, signal errors increase at the receiving end, which disables reception of signals. Explaining specifically, as illustrated in the lower drawing in FIG. 8 , when multiple stages of filters are concatenated, the filter bandwidth (150 GHz) 21 becomes narrow as indicated by a reference sign 23 . As a result of the phenomenon, there occurs a case where, since signals at both ends out of four optical signals 22 are trimmed (portions that are trimmed off are indicated by hatching), the level of the signals at both ends is decreased to a level at which the signals cannot be received.

Setting gap areas (that is, guard bands) between optical signals and both ends of the filter band in order to suppress such a phenomenon is conceivable. In this case, however, there is a problem in that the accommodation rate for optical signals is reduced by as much as the width of the set guard bands.

In order to solve such a problem, the optical transmission device according to the present example embodiment is configured in such a way that portions of the pass bands (that is, filter bands) of respective adjacent input ports of a cyclic AWG overlap each other. Explaining by use of a specific example, the optical transmission device is configured in such a way that a portion of the pass band of the port P 1 and a portion of the pass band of the port P 2 of the cyclic AWG AWG_ 1 illustrated in FIG. 4 overlap each other as pass bands 31 _ 1 and 31 _ 2 illustrated in FIG. 9 do (that is, an overlapping area 35 is set). In addition, the optical transmission device is configured in such a way that a portion of the pass band of the port P 2 and a portion of the pass band of the port P 3 of the cyclic AWG AWG_ 1 illustrated in FIG. 4 overlap each other as pass bands 31 _ 2 and 31 _ 3 illustrated in FIG. 9 do (that is, an overlapping area 36 is set).

For example, expanding the pass-band width of each of the input ports of a cyclic AWG while maintaining a frequency interval(s) between adjacent input ports thereof enables portions of the pass bands of the respective adjacent input ports to overlap each other.

The pass bands of the respective input ports of the cyclic AWG may be expanded so that an overlapping area(s) exist(s) between the pass bands of adjacent input ports by, for example, adjusting the shapes of input and output waveguides of the cyclic AWG. For example, the pass band of each input port of the cyclic AWG can be expanded by adjusting the shapes of a coupling portion between an input waveguide and a slab waveguide on the input side and a coupling portion between a slab waveguide on the output side and an output waveguide of the cyclic AWG.

Explaining by use of a specific example, as illustrated in FIG. 9 , while a frequency interval 34 is maintained at 150 GHz, the width of each of the pass bands 31 _ 1 to 31 _ 3 of the respective ports P 1 to P 3 is expanded from 150 GHz to 162.5 GHz. Setting the frequency interval and the pass-band width as described above enables a portion of the pass band 31 _ 1 of the input port P 1 and a portion of the pass band 31 _ 2 of the input port P 2 to overlap each other. In this case, the width of the overlapping area 35 where the pass band 31 _ 1 of the input port P 1 and the pass band 31 _ 2 of the input port P 2 overlap each other becomes 12.5 GHz. In addition, setting the frequency interval and the pass-band width as described above enables a portion of the pass band 31 _ 2 of the input port P 2 and a portion of the pass band 31 _ 3 of the input port P 3 to overlap each other. In this case, the width of the overlapping area 36 where the pass band 31 _ 2 of the input port P 2 and the pass band 31 _ 3 of the input port P 3 overlap each other becomes 12.5 GHz.

As described above, the optical transmission device according to the present example embodiment is configured in such a way that the pass bands 31 _ 1 to 31 _ 3 overlap each other between adjacent input ports of the cyclic AWG. Accordingly, optical signals 32 may be suppressed from being trimmed at both ends of the frequency band thereof during a transmission process of the optical signals 32 and an optical signal that cannot be received may be suppressed from occurring. Therefore, transmission characteristics may be suppressed from deteriorating. Meanwhile, although making pass bands overlap each other between adjacent input ports causes an optical signal to one input port of the adjacent input ports to leak into the other input port and crosstalk to occur, optimizing overlapping width between filters through a comparison between filter band narrowing and crosstalk so that deterioration of optical signals is minimized enables best transmission characteristics to be obtained.

The whole or part of the example embodiments disclosed above can be described as, but not limited to, the following supplementary notes.

(Supplementary Note 1)

An optical transmission device comprising

a cyclic AWG that filters respective optical signals inputted to each input port,

wherein the respective optical signals are constituted so that a plurality of wavelength-multiplexed signals can be allocated within one channel band,

the respective optical signals are filtered in channel units, and

pass-band width of each of the input ports of the cyclic AWG corresponds to bandwidth of each of the channels.

(Supplementary Note 2)

The optical transmission device according to supplementary note 1, wherein

pass-band width of each of the input ports of the cyclic AWG is set at m (m is a positive real number not less than 2) times of an adjacent frequency interval of the plurality of wavelength-multiplexed signals.

(Supplementary Note 3)

The optical transmission device according to supplementary note 2, wherein

pass-band width of each of the input ports of the cyclic AWG is 150 GHz, and a center frequency interval of the input ports is 150 GHz.

(Supplementary Note 4)

The optical transmission device according to supplementary note 3, wherein

the adjacent frequency interval is 37.5 GHz, and a value of the m is 4.

(Supplementary Note 5)

The optical transmission device according to supplementary note 3, wherein

the adjacent frequency interval is 50 GHz, and a value of the m is 3.

(Supplementary Note 6)

The optical transmission device according to supplementary note 3, wherein

the adjacent frequency interval is 75 GHz, and a value of the m is 2.

(Supplementary Note 7)

The optical transmission device according to any one of supplementary notes 1 to 6 comprising:

n (n is a natural number not less than 2) beam splitters each of which is connected to one of n input side paths; and

n cyclic AWGs that are arranged at a succeeding stage of the n beam splitters,

wherein each of the n beam splitters outputs respective optical signals that are inputted to the beam splitter to one of the input ports of each of the n cyclic AWGs,

each of the n cyclic AWGs is a cyclic AWG with n inputs and one output, and

each of the n cyclic AWGs filters the respective optical signals inputted to each of the input ports and outputs the filtered optical signals to an output side path.

(Supplementary Note 8)

The optical transmission device according to any one of supplementary notes 1 to 7, wherein

portions of pass bands of respective adjacent input ports of the cyclic AWG overlap each other.

(Supplementary Note 9)

The optical transmission device according to supplementary note 8,

by expanding pass-band width of each of the input ports of the cyclic AWG while maintaining a frequency interval between the adjacent input ports of the cyclic AWG, making portions of pass bands of the respective adjacent input ports overlap each other.

(Supplementary Note 10)

A transmission method of optical signals comprising,

with respect to each of input ports of a cyclic AWG in which pass-band width of each of the input ports corresponds to bandwidth of each channel:

inputting a plurality of channels of wavelength-multiplexed signals where a plurality of optical signals are wavelength-multiplexed within a band of each of the channels; and

filtering and outputting the wavelength-multiplexed signals in channel units.

(Supplementary Note 11)

The transmission method of optical signals according to supplementary note 10, wherein

pass-band width of each of the input ports of the cyclic AWG is set at m (m is a positive real number not less than 2) times of an adjacent frequency interval of the plurality of wavelength-multiplexed signals.

(Supplementary Note 12)

The transmission method of optical signals according to supplementary note 10 or 11 comprising:

outputting the wavelength-multiplexed signals inputted to each of n (n is a natural number not less than 2) beam splitters each of which is connected to one of n input side paths to one of the input ports of each of n cyclic AWG with n inputs and one output; and

filtering the wavelength-multiplexed signals inputted to each of the input ports and outputting the filtered optical signals to an output side path.

(Supplementary Note 13)

The transmission method of optical signals according to any one of supplementary notes 10 to 12, wherein

portions of pass bands of respective adjacent input ports of the cyclic AWG overlap each other.

(Supplementary Note 14)

The transmission method of optical signals according to supplementary note 13 comprising,

by expanding pass-band width of each of the input ports of the cyclic AWG while maintaining a frequency interval between the adjacent input ports of the cyclic AWG, making portions of pass bands of the respective adjacent input ports overlap each other.

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.

This application is based upon and claims the benefit of priority from Japanese patent application No. 2016-38162, filed on Feb. 29, 2016, the disclosure of which is incorporated herein in its entirety by reference.

1 , 2 Optical transmission device

11 Channel band

12 Wavelength-multiplexed signal

21 Filter bandwidth

22 Optical signal

31 _ 1 to 31 _ 3 Pass band (filter band)

32 Optical signal

34 Frequency interval

35 , 36 Overlapping area